1. Field of the Invention
The invention pertains to the field of variable cam timing. More particularly, the invention pertains to an oil pressure actuated variable cam timing phaser with oil pressure assist.
2. Description of Related Art
Apart from the cam torque actuated (CTA) variable camshaft timing (VCT) systems, the majority of hydraulic VCT systems operate under two principles—oil pressure actuation (OPA) or torsional assist (TA). In the oil pressure actuated VCT phaser, one or more operating segments 30 each include a vane 4 defining the operating segments 30 into first working chambers 2 and second working chambers 3 in fluid communication with an oil control valve (OCV) 9. In the OPA VCT phaser, the OCV directs engine oil pressure to the first working chamber 2 while simultaneously venting the second opposing working chamber 3 defined by the housing 1, the rotor 5, and the vane 4. This creates a pressure differential across one or more of the vanes 4 to hydraulically push the VCT phaser in one direction or the other. Neutralizing or moving the OCV 9 to a null position in which the OCV 9 blocks fluid flow into and out of the first and second working chambers puts equal pressure on opposite sides of the vane 9 and holds the phaser in position. If the phaser is moving in a direction such that valves will open or close sooner, the phaser is said to be advancing and if the phaser is moving in a direction such that valves will open or close later, the phaser is said to be retarding.
Conventional phasers have three, four, or five operating segments 30. Within each of the operating segments is a vane 4 separating the chamber 17 formed between the housing 1 and the rotor 5 into first working chambers 2 and second opposing working chambers 3, commonly referred to as advance chambers and retard chambers. In conventional phasers, supply oil pressure is provided to each side of all of the vanes 4, designated V1 V2, V3, V4.
Referring to FIG. 1, the housing assembly 1 of the phaser has an outer circumference 7 for accepting drive force. The rotor assembly 5 is connected to the camshaft and is coaxially located within the housing assembly 1. The rotor assembly 5 has a vane(s) 4 separating chamber(s) 17 formed between the housing assembly 1 and the rotor assembly 5 into an advance chambers 2, designated A1, A2, A3, A4 and a retard chambers 3, designated R1, R2, R3, R4. The vanes 4 are capable of rotation to shift the relative angular position of the housing assembly 1 and the rotor assembly 5.
An oil control valve 9 is in fluid communication with all of the advance chambers 2 and the retard chambers 3 through advance passages 12 and retard passages 13. The oil control valve 9 controls the flow fluid from supply pump 18 to all of the advance chambers 2 and retard chambers 3 and from the advance chambers 2 and retard chambers 3 to exhaust 19. The oil control valve 9 may be biased in a first direction by a spring 40 and a second direction by an actuator 42.
If the phaser were to be moving toward an advance position, supply oil pressure 18 would be provided to all of the advance chambers 2, designated A1, A2, A3, A4 of the phaser e.g. all three, four or any number of the advance chambers present in the phaser, and any oil pressure in the retard chambers 3, designated R1, R2, R3, R4 e.g. all three, four or any number of the retard chambers present in the phaser, would all be exhausted or vented 19.
Additionally, if the phaser were to moving towards a retard position, supply oil pressure 18 would be provided to all of the retard chambers 3 of the phaser, designated R1, R2, R3, R4 e.g. all three, four or any number of the retard chambers present in the phaser, and any oil pressure in the advance chambers 2, designated A1, A2, A3, A4 e.g. all three, four or any number of the advance chambers present in the phaser, would be exhausted or vented 19.
Additionally, the phaser may be held in a null position in which the supply oil pressure 18 to advance chambers 2 and the retard chambers 3 is blocked and oil within the chambers is prevented from exhausting.
The torsional assist (TA) systems operates under a similar principle as the OPA systems, with the exception that it has one or more check valves to prevent the VCT phaser from moving in a direction opposite than being commanded, should it incur an opposing force such as torque.
In some applications of oil pressure actuated phasers and torsional assist phasers in engines, a bias towards the advance position is necessary. The bias is usually achieved with a bias spring or set of bias springs. The bias springs may be present within the advance or retard chambers themselves or between the between the housing and the rotor to bias the phaser towards an advance position.